Growing Composites

Growing Composites

Bio-composites – fiber reinforced polymers made with bio-based resins and/or bio-based fiber reinforcement – are still very new materials, just beginning to appear in the architectural realm. We posted one such project a few months ago, but they are still rare.

In fact, it was the specific goal of proving the architectural value of these materials that moved the European Commission to fund the BioBuild project, a consortium of 13 companies from seven countries who worked together for three and a half years to design and fabricate four building systems made of bio-composites.

We spoke with Morten Lund, an Engineer in Architecture with GXN who was a project manager on the BioBuild Project. (GXN, one of the BioBuild consortium partners, was founded as an internal research group of the 3xn Architecture, but was spun off as a separate entity in 2012.) Lund explained that the point of the project was to use bio-based products as a direct substitute for existing products.

“You can buy the materials,” Lund points out, “but nobody’s really validated them for construction use. The whole scope of project was to prove that the materials have a viable use in construction.”

Four systems were selected as test cases for this proposition:

  • Exterior wall panel
  • Exterior cladding kit (rain-screen system)
  • Interior partition system
  • Suspended ceiling system

The Exterior Wall Panel, which won the JEC Innovation Award 2015 in the construction category, is a self-supporting glazed unit featuring a faceted design intended to provide partial shading of the window at critical times of day. It is 4m x 2.3m, weighs about 1200 kg, and comes pre-assembled and ready to install.

Materials

Lund explains that BioBuild products use two different resins and two different reinforcing fibers. Flax fiber is a high-performance material used for strength. Jute fiber, which is lower in both cost and embodied energy, is used as a filler material where structural strength is not as critical.

The bio-based fibers have somewhat different performance properties than more conventional glass and carbon fibers. “One of the good things about natural fibers,” comments Lund, “because they have a lower modulus of strength, they can bend much more. They are more compliant when made into a composite, they will bend more before they break. Potentially, a properly designed bio-composite would have a slower failure sequence, and you would be able to see they were cracking before they actually snapped.”

Strength of the composite depends on the type fiber and quality of that fiber. The strongest flax fibers were found to be comparable in strength to low end glass fiber.  Jute is about 1/2 – 1/10 the strength of glass fiber.  Composite strength is also very dependent, according to Lund, on the adhesion between fibers and matrix.  Natural fibers have shown very good adhesion, possibly compensating for the lower tensile strength.

The façade panel uses a bio-based polyester resin developed by DSM. “They actually have commercial grade resin available with part bio content. They supplied us with an experimental resin system that has higher bio content. It was about 85% – they won’t tell us exactly, but they said it’s the highest.”

A different resin was used on some of the other products, a furan-based material derived from a waste-product of sugar cane processing. “The really good thing about furan-based resins,” comments Lund “is that they are naturally fire-retardant. They act like phenolics. It charcoals, and that helps protect the rest of the composite. It has some intrinsic fire resistance, and we tried to use that, but to protect the fibers, we’ve had to use an intumescent coating that protects the fibers underneath.”

The façade panel was designed to show off both the architectural possibilities and the performance of the system. Lund notes that the structural performance and stiffness are increased by the depth of the design, but the design can be modified to suit architectural demands. For example, the design could be varied to accommodate the angle of the sun at the specific location of the building.

Polyester was selected for the façade panel because it is easier to work with. It can be formed using hand lay-up and vacuum infusion. “Furan needs to be pre-pregged and compression molded. The method we used was semi-continuous compression molding. You can produce very long piece, but it’s limited in profiles you can produce.”

Facade panel structure

The facade panel elements, (l-r): Glass pane, bio-composite exterior shell, wood fiber insulation, wooden substructure, bio-composite interior shell, aluminum interface.

The façade panel consists of bio-composite exterior shell, a layer of wood-fiber insulation, a wood substructure, and a bio-composite interior shell. An aluminum edge-frame provides an interface to the surrounding interior wall.

“The wooden structure is there to ensure that the composite is not stressed unnecessarily. From an engineering point of view, we wanted to make sure this first design was fully sufficient.  With a bit more testing and validation from fiber manufacturers, we could probably remove the intermediate wood frame. The composite surface is taking quite a bit of the surface loads, that is, wind and impact loads.”

The rain-screen cladding system is made from jute and flax fibers with the furan-based binder. The continuous compression-molded panel has a shape on the back that allows it to span further than normal rain-screen cladding. The current version is 2 m long, but could potentially go to 2.5m, and with the proper molding machine, could be as tall as 1 meter high. It is designed to attach similarly to an aluminum panel system.

The suspended ceiling system is made of jute fiber and polyester. The lamellas are acoustically open, with acoustic-absorbent material on the top side. “These lamellas would allow for high degree of freedom in design,” notes Lund, including the ability to make curved ceilings.

The interior partition system is designed as a sliding-wall system for offices and meeting rooms. It could easily be adapted as a fixed-wall system, too. “The main point,” explains Lund, “was that they could be manufactured offsite and brought in quickly.”

Why Bio?

While all this is very intriguing and exciting, there is one aspect of the program that I found puzzling, and I was glad of the opportunity to question Lund about it.   I could easily see the attraction of bio-resins, since they are derived from renewable sources, unlike conventional resins that are largely petroleum-based. But I wondered why anyone would want to use bio-based fibers, since glass fiber is stronger and is made from silica, the second most abundant mineral on the planet.

It turns out my basic assumptions were all wrong. The point of this program was not the renewability of the raw material sources, but the embodied energy of the material and the end-of-life disposal of the material.

“The main issue with using composites is a waste issue,” explains Lund. “Construction produces a significant part of waste streams. I think in Denmark, 20% of all waste comes from construction industry.

“In Europe, we’re shifting to a focus on embodied energy. Composites have a lot of potential. The problem comes as soon as you make a composite using plastic, you have a disposal problem, a landfill problem. We at GXN have been working quite a bit with cradle to cradle. Instead of going into a technical cycle where you would recycle material into the same space, we tried to go into the natural cycle, recuperating that energy through incineration at the end of the life cycle.”

In other words, if you can burn the composite at the end of its service life, and capture that heat for beneficial use, then you have effectively reduced the embodied energy of the composite, and simultaneously avoided the landfill problem. Resins will burn.  Glass reinforcement will not burn, but its material properties are severely impacted, so it cannot be reused, either. Bio-based reinforcement, by contrast, burns well, allowing a higher percentage of the embodied energy to be recaptured an reducing landfill impacts.

Right Now, and Going Forward

These applications of the materials, and the materials themselves, are still in the prototype phase, and not surprisingly, are currently very expensive. The bio-fiber based composites have about half the weight of glass-fiber composite, so some of the material expense is potentially offset by lower installation costs. Full-scale manufacturing would also, potentially, lower the cost significantly compared to the labor-intensive methods used for the prototypes. “But right now, it’s expensive, on verge of being prohibitively expensive in materials.”

This effort comes at a time when composites are still emerging in the construction industry. There are currently no European regulations for using composites in construction, but the codes are in the works. “If we can have a viable substitute before we have a European regulation on using composite in construction, we have a better chance,” says Lund. To that end, they are testing actively – fire testing, thermal testing, mechanical testing – “all to the same standard as normal CE marking for Euro standards. The people involved in testing the products are part of the technical network involved in writing the euro codes for composites.”

The 13 companies in the BioBuild consortium are ACCIONA Infrastructures (Spain), Amorim Cork Composites (Portugal), Arup Deutschland (Germany), Exel Composites (UK), Fiber-Tech Construction (Germany), GXN Innovation (Denmark), Institut für Verbundwerkstoffe Kaiserslautern (Germany), Katholieke Universiteit Leuven (Belgium), Laboratorio Nacional De Engenharia Civil (Portugal), NetComposites (UK), SHR (Netherlands), Nederlandse Organisatie voor Togepast TNO (Netherlands).

Images via GXN.